.\" Copyright (c) 1983 Regents of the University of California.
.\" All rights reserved.  The Berkeley software License Agreement
.\" specifies the terms and conditions for redistribution.
.\"
.\"	@(#)intro.4n	6.3 (Berkeley) 8/1/87
.\"
.TH INTRO 4N "August 1, 1987"
.UC 5
.SH NAME
networking \- introduction to networking facilities
.SH SYNOPSIS
.nf
.ft B
#include <sys/socket.h>
#include <net/route.h>
#include <net/if.h>
.fi R
.fi
.SH DESCRIPTION
.de _d
.if t .ta .6i 2.1i 2.6i
.\" 2.94 went to 2.6, 3.64 to 3.30
.if n .ta .84i 2.6i 3.30i
..
.de _f
.if t .ta .5i 1.25i 2.5i
.\" 3.5i went to 3.8i
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..
This section briefly describes the networking facilities
available in the system.
Documentation in this part of section
4 is broken up into three areas:
.I "protocol families
(domains),
.IR protocols ,
and
.IR "network interfaces" .
Entries describing a protocol family are marked ``4F,''
while entries describing protocol use are marked ``4P.''
Hardware support for network interfaces are found
among the standard ``4'' entries.
.PP
All network protocols are associated with a specific
.IR "protocol family" .
A protocol family provides basic services to the protocol
implementation to allow it to function within a specific
network environment.  These services may include 
packet fragmentation and reassembly, routing, addressing, and 
basic transport.  A protocol family may support multiple
methods of addressing, though the current protocol implementations
do not.  A protocol family is normally comprised of a number
of protocols, one per
.IR socket (2)
type.  It is not required that a protocol family support
all socket types.  A protocol family may contain multiple
protocols supporting the same socket abstraction. 
.PP
A protocol supports one of the socket abstractions detailed
in
.IR socket (2).
A specific protocol may be accessed either by creating a
socket of the appropriate type and protocol family, or
by requesting the protocol explicitly when creating a socket.
Protocols normally accept only one type of address format,
usually determined by the addressing structure inherent in
the design of the protocol family/network architecture.
Certain semantics of the basic socket abstractions are
protocol specific.  All protocols are expected to support
the basic model for their particular socket type, but may,
in addition, provide non-standard facilities or extensions
to a mechanism.  For example, a protocol supporting the
SOCK_STREAM
abstraction may allow more than one byte of out-of-band
data to be transmitted per out-of-band message.
.PP
A network interface is similar to a device interface.
Network interfaces comprise the lowest layer of the
networking subsystem, interacting with the actual transport
hardware.  An interface may support one or more protocol
families and/or address formats.
The SYNOPSIS section of each network interface
entry gives a sample specification
of the related drivers for use in providing
a system description to the
.IR /sys/conf/config
script.
The DIAGNOSTICS section lists messages which may appear on the console
and/or in the system error log,
.I /usr/adm/messages
(see
.IR syslogd (8)),
due to errors in device operation.
.SH PROTOCOLS
The system currently supports the DARPA Internet
protocols and the Xerox Network Systems(tm) protocols.
Raw socket interfaces are provided to the IP protocol
layer of the DARPA Internet, to the IMP link layer (1822), and to
the IDP protocol of Xerox NS.
Consult the appropriate manual pages in this section for more
information regarding the support for each protocol family.
.SH ADDRESSING
Associated with each protocol family is an address
format.  The following address formats are used by the system (and additional
formats are defined for possible future implementation):
.sp 1
.nf
._d
#define	AF_UNIX	1	/* local to host (pipes, portals) */
#define	AF_INET	2	/* internetwork: UDP, TCP, etc. */
#define	AF_IMPLINK	3	/* arpanet imp addresses */
#define	AF_PUP	4	/* pup protocols: e.g. BSP */
#define	AF_NS	6	/* Xerox NS protocols */
#define	AF_HYLINK	15	/* NSC Hyperchannel */
.fi
.SH ROUTING
The network facilities provided limited packet routing.
A simple set of data structures comprise a ``routing table''
used in selecting the appropriate network interface when
transmitting packets.  This table contains a single entry for
each route to a specific network or host.  A user process,
the routing daemon, maintains this data base with the aid
of two socket-specific 
.IR ioctl (2)
commands, SIOCADDRT and SIOCDELRT.  The commands allow
the addition and deletion of a single routing
table entry, respectively.  Routing table manipulations may
only be carried out by super-user.
.PP
A routing table entry has the following form, as defined
in
.RI < net/route.h >;
.sp 1
._f
.nf
struct rtentry {
	u_long	rt_hash;
	struct	sockaddr rt_dst;
	struct	sockaddr rt_gateway;
	short	rt_flags;
	short	rt_refcnt;
	u_long	rt_use;
	struct	ifnet *rt_ifp;
};
.sp 1
.fi
with
.I rt_flags
defined from,
.sp 1
.nf
._d
#define	RTF_UP	0x1		/* route usable */
#define	RTF_GATEWAY	0x2		/* destination is a gateway */
#define	RTF_HOST	0x4		/* host entry (net otherwise) */
#define	RTF_DYNAMIC	0x10		/* created dynamically (by redirect) */
.fi
.PP
Routing table entries come in three flavors: for a specific
host, for all hosts on a specific network, for any destination
not matched by entries of the first two types (a wildcard route). 
When the system is booted and addresses are assigned
to the network interfaces, each protocol family
installs a routing table entry for each interface when it is ready for traffic.
Normally the protocol specifies the route
through each interface as a ``direct'' connection to the destination host
or network.  If the route is direct, the transport layer of
a protocol family usually requests the packet be sent to the
same host specified in the packet.  Otherwise, the interface
is requested to address the packet to the gateway listed in the routing entry
(i.e. the packet is forwarded).
.PP
Routing table entries installed by a user process may not specify
the hash, reference count, use, or interface fields; these are filled
in by the routing routines.  If
a route is in use when it is deleted
.RI ( rt_refcnt
is non-zero),
the routing entry will be marked down and removed from the routing table,
but the resources associated with it will not
be reclaimed until all references to it are released. 
The routing code returns EEXIST if
requested to duplicate an existing entry, ESRCH if
requested to delete a non-existent entry,
or ENOBUFS if insufficient resources were available
to install a new route.
User processes read the routing tables through the
.I /dev/kmem 
device.
The
.I rt_use
field contains the number of packets sent along the route.
.PP
When routing a packet,
the kernel will first attempt to find a route to the destination host.
Failing that, a search is made for a route to the network of the destination.
Finally, any route to a default (``wildcard'') gateway is chosen.
If multiple routes are present in the table,
the first route found will be used.
If no entry is found, the destination is declared to be unreachable.
.PP
A wildcard routing entry is specified with a zero
destination address value.  Wildcard routes are used
only when the system fails to find a route to the
destination host and network.  The combination of wildcard
routes and routing redirects can provide an economical
mechanism for routing traffic.
.SH INTERFACES
Each network interface in a system corresponds to a
path through which messages may be sent and received.  A network
interface usually has a hardware device associated with it, though
certain interfaces such as the loopback interface,
.IR lo (4),
do not.
.PP
The following 
.I ioctl
calls may be used to manipulate network interfaces.
The
.I ioctl
is made on a socket (typically of type SOCK_DGRAM)
in the desired domain.
Unless specified otherwise, the request takes an
.I ifrequest
structure as its parameter.  This structure has the form
.PP
.nf
.DT
struct	ifreq {
#define	IFNAMSIZ	16
	char	ifr_name[IFNAMSIZ];		/* if name, e.g. "en0" */
	union {
		struct	sockaddr ifru_addr;
		struct	sockaddr ifru_dstaddr;
		struct	sockaddr ifru_broadaddr;
		short	ifru_flags;
		int	ifru_metric;
		caddr_t	ifru_data;
	} ifr_ifru;
#define	ifr_addr	ifr_ifru.ifru_addr	/* address */
#define	ifr_dstaddr	ifr_ifru.ifru_dstaddr	/* other end of p-to-p link */
#define	ifr_broadaddr	ifr_ifru.ifru_broadaddr	/* broadcast address */
#define	ifr_flags	ifr_ifru.ifru_flags	/* flags */
#define	ifr_metric	ifr_ifru.ifru_metric	/* metric */
#define	ifr_data	ifr_ifru.ifru_data	/* for use by interface */
};
.fi
.TP
SIOCSIFADDR
Set interface address for protocol family.  Following the address
assignment, the ``initialization'' routine for
the interface is called.
.TP
SIOCGIFADDR
Get interface address for protocol family.
.TP
SIOCSIFDSTADDR
Set point to point address for protocol family and interface.
.TP
SIOCGIFDSTADDR
Get point to point address for protocol family and interface.
.TP
SIOCSIFBRDADDR
Set broadcast address for protocol family and interface.
.TP
SIOCGIFBRDADDR
Get broadcast address for protocol family and interface.
.TP
SIOCSIFFLAGS
Set interface flags field.  If the interface is marked down,
any processes currently routing packets through the interface
are notified;
some interfaces may be reset so that incoming packets are no longer received.
When marked up again, the interface is reinitialized.
.TP
SIOCGIFFLAGS
Get interface flags.
.TP
SIOCSIFMETRIC
Set interface routing metric.
The metric is used only by user-level routers.
.TP
SIOCGIFMETRIC
Get interface metric.
.TP
SIOCGIFCONF
Get interface configuration list.  This request takes an
.I ifconf
structure (see below) as a value-result parameter.  The 
.I ifc_len
field should be initially set to the size of the buffer
pointed to by 
.IR ifc_buf .
On return it will contain the length, in bytes, of the
configuration list.
.PP
.nf
.DT
/*
 * Structure used in SIOCGIFCONF request.
 * Used to retrieve interface configuration
 * for machine (useful for programs which
 * must know all networks accessible).
 */
struct	ifconf {
	int	ifc_len;		/* size of associated buffer */
	union {
		caddr_t	ifcu_buf;
		struct	ifreq *ifcu_req;
	} ifc_ifcu;
#define	ifc_buf	ifc_ifcu.ifcu_buf	/* buffer address */
#define	ifc_req	ifc_ifcu.ifcu_req	/* array of structures returned */
};
.fi
.SH SEE ALSO
socket(2),
ioctl(2),
intro(4),
config(8),
routed(8C)
